Minimally invasive medical devices that navigate natural body lumens need to be small enough to fit within the lumens. Lung catheters, for example, which may be used to perform minimally invasive lung biopsies or other medical procedures, may need to follow airways that decrease in size as the catheter navigates branching passages. To reach a target location in a lung, a catheter may follow passages having diameters as small as 3 mm or less. Manufacturing a catheter that is sufficiently small and includes the mechanical structures and sensors for remote or robotic operation can be challenging.
Electromagnetic sensors (EM) sensors can measure the position and orientation of a portion of a medical instrument. EM sensors are particularly suitable for minimally invasive medical instruments because EM sensors can combine high global accuracy with a small diameter package size. During EM sensor operation, a generator external to a patient can produce a well-controlled, time-varying magnetic field, and in response, one or more coils of an EM sensor in or on a portion of the medical instrument produce induced electrical signals. In particular, time variations in the magnetic field induce currents in the coils of the EM sensor, and the pose of each coil can be partially determined from knowledge of the generated magnetic field and the geometry of the coil. A single coil can be used, for example, to measure a position and a pointing direction, e.g., pitch and yaw angles, but a cylindrically symmetrical coil is unable to distinguish roll angles about the symmetry axis of the coil. Accordingly, EM sensors employing a single cylindrical coil have been used as 5-Degree-of-Freedom (5-DoF) sensors. To additionally measure the roll angle, a 6-DoF EM sensor generally requires two coils having symmetry axes that are not parallel, e.g., perpendicular symmetry axes.
The long, thin shape typical of 5-DoF EM sensors fits well with minimally invasive medical instruments or tools, which often have long and thin extensions. However, with the central axis of a single coil sensor aligned with the roll axis of an instrument, such 5-DoF EM sensors cannot measure the roll angle of the instrument. While some symmetric instruments such as needles may not require roll angle measurements, many instruments require knowledge of the roll angle of the instrument, particularly for robotic control. Measurement of the roll angle may require a 6-DoF sensor that includes two coils. For example, to create a 6-DoF EM sensor, two 5-DoF EM sensors may need to be placed perpendicular or at a non-zero angle to each other, which creates a much larger sensor package. If each 5-DoF sensor has a cylindrical shape about 1 mm in diameter and about 10 mm long, the 6-Dof sensor containing two 5-DoF sensors may be up to 10×10×1 mm. While the 1 mm diameter of a 5-DoF EM sensor may fit within a small, e.g., 3 mm diameter, instrument, a 10-mm wide 6-DoF EM sensor may not fit in a small instrument.